We use Exclamations to express surprise, anger, joy and so on:

What an idiot!

Unbelievable!

How fantastic!

As you can see, in writing an exclamation is shown using an Exclamation Mark (or Exclamation Point) which is a vertical line over a dot. See Exclamation Mark for more on this.

An exclamation then is often an interjection – something suddenly expressed to show emotion.

Exclamation Phrases

There are a few main forms of exclamation phrases which are common in English:

{what} + {non-countable noun phrase}
What luck!

{what} + {article} + {countable noun phrase}
What a lunatic!

{how} + {adjective}
How incredible!
How curious!

When we use a question word (what, how, etc) in an exclamation, we do not invert the subject and verb‏‎ as we would in a normal question:

Normal Question: How big is it?
Exclamation: Look how big it is!